R_{1} = \frac{U_{0}^{2}}{P_{0}} = \frac{3, 6^{2}}{1, 8} = 7, 2 Ω

${R_1 = \frac{U_0^2}{P_0}=\frac{3,6^{2}}{1,8}=7,2\,\mathrm{\Omega}}$

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